Several medical imaging techniques are now currently in use to investigate the severity of vascular diseases (i.e. quantification of the vascular lumen geometry) and enable clinicians to detect stenoses, thromboses, development of collateral vessels, aneurysms, or malformations. The techniques are based either on X-rays (X-ray angiography, and computerized tomography (CT)), ultrasonography (B-mode, M-mode, pulsed-wave Doppler, power Doppler, color Doppler, intravascular ultrasound (IVUS)), or on magnetic resonance angiography (MRA) (gradient-recalled echo sequence, phase-contrast, gadolinium enhanced angiography). Angiography (MRA) provides geometrical data on the vessel lumen, whereas IVUS and CT can be used independently or complementary to angiography to investigate the arterial wall morphology and composition. Knowledge on the hemodynamics is also of great interest to evaluate the consequences of lesions on blood supply to the tissues perfused by diseased vessels. Doppler ultrasound and phase contrast MRA allow to study blood flow, namely to measure blood velocities in the vessels. As the precise quantification of morphological and hemodynamic parameters is the basis of the clinical diagnosis, calibration of the medical imaging apparatuses is an essential step required for accurate imaging and evaluation of blood vessels. Test objects, known as calibration phantoms, are commonly used for this purpose and specific phantoms have been developed to meet the requirements associated to each imaging modality.
Even after calibration, no imaging technique is error free. In the literature, plane X-ray angiography is considered as the gold standard (Bendib K., Poirier C., Croisille P., Roux J. P., Revel D., and Amiel M.—Caractérisation d'une sténose artérielle par imagerie 3D, Journal de Radiologie, 1999, 80:1561-1567) for the evaluation of arterial diseases, because it is the technique with the best spatial resolution. Nevertheless, other techniques, especially those allowing 3D imaging, bring important additional information concerning the morphology, the severity, and the location of the lesion. This is why comparative studies of imaging techniques, in the same experimental conditions, are necessary to assess the accuracy and determine the advantages and limitations of each one. Moreover, a gold standard, different from the tested techniques, should be available for precise assessment.
Vascular flow phantoms are ideal tools for such studies since they provide a way of testing the geometric accuracy, with easy reproducibility of the experimental conditions when different modalities are tested. They can also be used to compare the blood flow velocity patterns obtained by ultrasound and MRA. Moreover, it is possible to reproduce vascular pathologies, with a known geometry that can be accurately determined during fabrication, and which can be used as the “gold standard reference” for evaluation of imaging devices. Multimodality phantoms have to meet three major requirements. First, they must be compatible with many if not all the imaging modalities evaluated, i.e. it is necessary that the vessel position can be clearly identified on the images, with no or minimum artifacts in any modality. Second, they should be anthropomorphic, i.e. their geometry should mimic as close as possible the complexity of real human vessels. Finally, they should contain markers visible in all modalities for image calibration, resealing and fusion.
Multimodality anthropomorphic vascular flow phantoms have been proposed in the recent years using three major techniques: stereolithography, phantoms including real vessels and lost-material casting method. For instance, Creasy et al. (Creasy J. L., Crump D. B., Knox K., Kerber C. W., and Price R. R.—Design and Evaluation of a Flow Phantom, Academic radiology, 1995, 2:902-904) presented a simple cranial blood flow phantom compatible with X-ray, MRA and CT angiography. It consisted in an acrylic skull filled with a silicone polymer mimicking human brain tissue, which contains the main cerebral vessels. Arteries were modeled from actual human arteries by injecting fresh cadaver arteries with acrylic resin. Veins were constructed in wax using resin cast human model duplicating dimensions and shape of actual cerebral human veins. When the vein and artery models were placed and the skull filled with silicon polymer, wax was removed thermically and chemically. Fahrig et al. (Fahrig R., Nikolov H., Fox A. J., and Holdsworth D. W.—A Three-Dimensional Cerebrovascular Flow Phantom, Medical Physics, 1999, 26(8):1589-1599) constructed a three-dimensional cerebrovascular flow phantom compatible with X-ray angiography, MRA and CT techniques using data taken from the literature and a casting method similar to that described above and cerrolow 117 as the casting material. The authors tested the phantom for geometric accuracy using high resolution MRA and CT protocols. Their results showed good agreement (within 4%) between the arterial diameters determined from the radiographic images and those measured on cerrolow cores before their implantation.
To solve the problem of realistic anthropomorphic geometry, including diseased segments, studies have been made on phantoms derived from real vessels harvested on cadavers (Kerber C. W., and Heilman C. B.—Flow Dynamics in the Human Carotid Artery: I. Preliminary Observations Using a Transparent Elastic Model, American Journal of Neuroradiology, 1992, 13:173-180). Dabrowski et al. (Dabrowski W., Dunmore-Buyze J., Rankin R. N., Holdsworth D. W., and Fenster A.—A real vessel phantom for imaging experimentation, Medical Physics, 1997, 24(5):687-693) used a human abdominal aorta, fixed with a 10% formaldehyde solution at 90 mmHg to preserve its geometry, to perform comparisons of X-ray angiography, CT scan and 3D B-mode ultrasound. The images obtained from the three modalities could be compared with each other and showed good overall correlation. These real vessel phantoms had two limitations: first, the geometry of the artery was not known a priori, and thus, there was no gold standard to assess the accuracy of the imaging devices. Second, the geometry of each artery was unique and could not be duplicated if the vessel was damaged.
Frayne et al. (Frayne R., Gowman L. M., Rickey D. W., Holdsworth D. W., Picot P. A., Drangova M., Chu K. C., Caldwell C. B., Fenster A., and Rutt B. K.—A Geometrically Accurate Vascular Phantom for Comparative Studies of X-Ray, Ultrasound, and Magnetic Resonance Vascular Imaging: Construction and Geometrical Verification, Medical Physics, 1993, 20(2):415-425) built a flow phantom of the human carotid bifurcation based on geometrical data taken from the literature by using a thin-walled polyester-resin replica of the carotid bifurcation surrounded by an agar tissue-mimicking material (lost-material casting technique). The two-parts mold was machined in blocks of acrylic using a numerical milling machine and the casting material was wax. The blood- and tissue-mimicking materials had X-ray, ultrasound and MRA properties close to those of blood and human tissues, but polyester resin was found to be a poor ultrasound and MRA tissue-mimicking material. Static images were recorded with X-ray angiography, CT, ultrasound and MRA for evaluation of the geometric accuracy of these techniques. Velocity images were acquired under steady flow with color Doppler and phase contrast MRA. The two techniques gave flow patterns which qualitatively agreed with each other and with literature data, and measured volume flow-rates were in good agreement (4.4%) with actual values.
Smith et al. (Smith R. F., Frayne R., Moreau M., Rutt B. K., Fenster A., and Holdsworth D. W.—Stenosed Anthropomorphic Vascular Phantoms for Digital Substraction Angiography, Magnetic Resonance and Doppler Ultrasound Investigations, SPIE Physics of medical imaging, 1994, 2163:235-242) improved the method proposed by Frayne et al. (1993) by using aluminum molds, replacing wax with cerrobend 158 and agar gel with a polyester resin. A drawback of this method is the absence of tissue-mimicking material around the vessel, which has implications in MRA and ultrasound images. Recently, Bendib et al. (1999) used vascular phantoms to compare the accuracy of MRA, CT angiography and 3D X-ray digital substraction angiography for evaluation of stenoses using stereolithography. One limitation of stereolithography is that it only allows fabrication of rigid-wall phantoms, and the type of materials that can be used is limited. Moreover, the lumen of the vessel is not perfectly smooth (Fahrig et al., 1999). The phantoms were filled with contrast agents compatible with each imaging modality, but there was no fluid circulation. The authors found that among the three methods tested, 3D X-ray angiography was more accurate than MRA and CT for the evaluation of the degree, the shape and the location of stenoses.
Also known in the art, there are the following U.S. Pat. Nos. 4,331,021; 4,499,375; 4,551,678; 4,644,276; 4,724,110; 4,794,631; 4,843,866; 4,985,906; 5,312,755; 5,560,242; and 5,793,835.
However, all of these patents describe apparatus and methods that are each limited to a single mode of imaging.
There is a need for a phantom using different modes of imaging like X-ray, ultrasound and magnetic resonance (MR) to calibrate apparatuses.